It is proposed to continue and extend a project on long range excitation transport and excitation penetration in semiordered molecular aggregates and interfaces. Excitation percolation transitions (catastrophic changes of transport or diffusion properties) will be studied for bulk molecular aggregates and simple organic interfaces, including transport on, through, to and from thin molecular films deposited on appropriate molecular crystal surfaces. Methods will be developed for the careful production of these interfaces, for the characterization of their structural properties, site heterogeneity and surface roughness as well as for the study of their excitations and excitation propagation. A method of "sandwiching" molecular and biomolecular layers, films and membranes with thin organic exciton conductor coatings ("excitation sandwiches") will also be developed. Naphthalenes or "inert" porphyrins will be used for coating the less stable systems, such as molecular chlorophyll deposits from solution or natural stacks of chlorophyll. Phosphorescence, fluorescence, delayed fluorescence, and resonance Raman techniques will be utilized to monitor exciton migration, trapping, detrapping, fusion and fission, and in particular spatial and spectral diffusion, using tunable laser excitation with nano-second time resolution, milli-Angstrom spectral resolution and micron spatial resolution, down to 1.5K, in conjunction with optional externally produced electric field gradients and mechanical stretching or compassion of film substrates. Theoretical formalisms, incorporating efficient computer simulation studies, will be applied to the percolation-limited migration kinetics of excitations, ions and molecules that "diffuse" in membrane-like heterogeneous media, emphasizing the critical (catastrophic) aspects. These studies should result in detailed information on the switching ("on and off") of the excitation transport in the studied prototype systems, as well as for in-vivo aggregates, under both normal and abnormal conditions. Short range applications involve the primary physical processes of photosynthesis and neuron excitation. The long range goal is to provide a model for a "computer" based on molecular excitons and its application to damaging processes affecting the brain and nervous system.